Method of enhancing powder compactibility

ABSTRACT

Method of improving compactibility and sintering characteristics of spherical metal powders, comprising selectively removing the less chemically resistant surface regions of the powders, to roughen the powder surfaces and thereafter compacting and sintering the thus-roughened powders.

United States Patent [1 1 Church et al.

1 1 Jan. 7, 1975 METHOD OF ENHANCING POWDER COMPACTIBILITY [75]Inventors: Nathan Lewis Church, Warwick,

N.Y.; Edwin Snape, Marton, England [73] Assignee: The InternationalNickel Company,

Inc., New York, NY.

[ 1 Filed: Dec.30, 1971 1 11 Appl. No.: 214,444

[52] US. Cl 75/211, 75/212, 75/214 [51] Int. Cl. B22f 1/00, B22f 3/12[58] Field of Search 75/211, 212, 214

[56] References Cited UNITED STATES PATENTS 1,913,133 6/1933 Stout75/212 Wulff 75/211 Butcher et al. 75/212 Primary ExaminerBenjamin R.Padgett Attorney, Agent, or FirmEwan C. MacQueen; Raymond .1. Kenny [57]ABSTRACT 11 Claims, No Drawings METHOD OF ENHANCING POWDERCOMPACTIBILITY -The present invention relates to a method of improvingthe compactibility of spherical metal powders and more particularly to amethod of chemically attacking selected surface portions of suchpowders.

Spherical pre-alloyed powders produced by gas atomization exhibit poorcompactibility. Such powders have high strength resulting from the alloycomposition and the rapid quench inherent in atomization, such thatrelatively little deformation of the powder can be attained inconventional powder metallurgy cold pressing operations, even whencompacting pressures as high as about 40 or even 70 tons per square inchare resorted to. In the case of spherical stainless steel powders thepressing problem is so severe that it is even difficult to form suchpowders into a compressed mass having sufficient green strength toenable removal of the pressed object from the die as a unitary piece. Asa result of such poor compactibility, it is necessary to employ adhesivebinders and undesirably high sintering temperatures to produce powdermetallurgical products from the spherical gas-atomized powders. Theresulting production difficulties and poor final properties seriouslydetract from the commercial desirability of such spherical gas-atomizedpowders.

Pre-alloyed, spherical powders exhibiting improved compactibility can beproduced by water atomization at relatively low water pressure. However,such wateratomized powders contain high levels of deleterious impuritiessuch as oxygen and oxides. Accordingly, a high-pressure wateratomization can be used to produce irregularly shaped, pre-alloyedpowders which exhibit relatively good compactibility and have relativelylow oxide contents. However, powder metallurgy products made from suchpowders exhibit excessive porosity due to the irregular shape of thepowders. It is necesary to employ long sintering times at hightemperatures, e.g., 2 to 4 hours at 2,300F., to close these pores, suchlonger times and higher temperatures being commercially undesirable andnot always effective.

It has now been discovered that a special process of treating sphericalpre-alloyed powders, such as those" produced by gas atomization,provides significant improvement in the compactibility thereof, withaccompanying improvement in sinterability. Powder metallurgy productsmade from such treated powders exhibit improved sintered densitieswithout the necessity of long sintering times or high sinteringtemperatures.

It is therefore, an object of the present invention to provide a methodof improving the compactibility of pre-alloyed metal powders having agenerally spherical shape.

Another object of the invention is to provide a method of improving thecompactibility and sinterability of pre-alloyed spherical powderswithout introducing therein deleterious oxygen or oxides.

A further object is to provide substantially spherical powders that canbe converted to relatively highdensity products with relatively lowcompacting pressures and sintering temperatures.

Generally speaking, the present invention comprises subjecting sphericalpowder particles, particularly those of stainless steel compositions, toselective chemical attack such that substantially only the lesschemically resistant surface regions are removed, i.e., selectivelyremoved, so as to roughen the powder surfaces.

The chemical attack can be achieved by subjecting the powder surfaces tothe action of a corrodent, which can be an acid or an alkali and whichis usually in the liquid state, the particular corrodent depending onthe material being treated. The selective chemical attack providesprojections or asperities at the powder surfaces.

The depth of removal depends on the corrodent and the time for whichchemical attack is carried out, it generally being required that thedepth of selective chemical attack be sufficient to weaken the surfacelayers of the particles so that they can be readily deformed. Forexample, with powder having an average particle diameter of about 40 to400 microns, the depth of selective attack can be about 1 to about 20microns, height of the resulting surface asperities or projectionsgenerally being on the order of such depth of attack. After theselective chemical attack, the powders may be rinsed and dried andthereafter compacted at a pressure of, e.g., about 10 to tons per squareinch to provide compacts having green densities of, e.g., about 65% toabout of theoretical. The green compact can then be sintered, e.g., atabout 1,800F. to about 2,100F. or even higher, to provide relativelyhigh density products, this without the need for additives for improvingsinterability.

The spherical powder particles can be selectively attacked chemically inthe as-atomized condition or preparatory steps can be taken to renderthe particles more susceptible to subsequent selective chemical attack,e.g., by providing two or more metallurgical phases at the surfaceportions of the particles.

Stainless steel powder treatable in accordance with the inventiongenerally contains, by weight, about 12 to 35% chromium, up to about 30%nickel, up to about 0.5% carbon, up to about 0.5% oxygen, up to about0.2% nitrogen, up to about 0.4% sulphur, up to about 3% copper, up toabout 0.4% phosphorus, up to about 2% silicon, up to about 10%manganese, up to about 15% cobalt, up to about 10% molybdenum, up toabout 5% tungsten and the balance iron and incidental impurities.

In one embodiment, stainless steel powder of appropriate composition canbe heat treated to render such powders more susceptible to selectivechemical attack. Such heat treatment can be employed to produce in thepowders one or more active metallurgical phases which can be metallic,e.g., martensite, ferrite and/or austenite. Generally, where thespherical powders contain two or more metallurgical phases, the lesschemically resistant phase can comprise, e.g., about 10 to 30 volume ormore of the powder. For example, with spherical stainless steel powderscontaining by weight, about 18 to 35% chromium, 2 to 12% nickel, up toabout 0.2% carbon, up to about 2% manganese, up to about 2% silicon, upto about 2% cobalt, up to about 3% molybdenum, up to about 5% tungsten,and the balance essentially iron, such heat treatment can be carried outby annealing the powder at a temperature of, for example, about l,600 to2,200F. and, more specifically, about l,800F., to 'obtain an austenitephase dispersed in a ferritic matrix. The annealing preferably iscarried out such that the dispersed austenitic phase is of relativelysmall size and is substantially uniformly distributed throughout theparticle surface portions. The selective chemical attack on thethus-treated powders can then be achieved by immersing the powderparticles in a corrodent, e.g., boiling aqueous sulfuric acid solution,so as to dissolve the surface regions comprising the less chemicallyresistant phase thereof. These surface regions preferably are removed toa depth of about to about microns where the average powder size is about50 to about 150 microns. Thereafter, the powder can be washed, e.g., inwater or alcohol, and dried, and subsequently compacted with relativeease, e.g., at 30 t.s.i. pressure, and sintered at, for example, about2,000 to 2,l00 F. in a hydrogen atmosphere.

In accordance with another embodiment spherical stainless steel powderhaving the composition, by weight, about to 35% chromium, about 0 to 24%nickel, up to about 0.2% carbon, up to about 2% manganese, up to about2% silicon, up to about 2% carbon, up to about 3% molybdenum, up toabout 5% tungsten, and the balance iron and incidental impurities can beprovided with a sigma phase dispersed in a matrix of austenite orferrite by heat treating the powder at about l,200 to 1,700F.Thereafter, the powders can be selectively chemically attacked byimmersing the powder particles in a suitable corrodent, e.g., aqueous70% nitric acid at 70C., so as to dissolve the less chemically resistantsigma phase regions located at the surface portions of the powderparticles. Then the thus-treated powders can be rinsed, e.g., in water,dried, compacted at, e.g., 10 to 70 t.s.i., and sintered at about2,000F., for example.

According to a further embodiment, spherical powder of austeniticstainless steel composition having the composition, by weight, of about12 to chromium, about 12 to nickel, up to about 1% silicon, up to about1.5% molybdenum, up to about 0.2% carbon, up to about 2% tungsten, up toabout 2% manganese, up to about 2% cobalt, and the balance iron andincidental impurities and having, for example, an austenitic structure,can be selectively chemically attacked by treating the powder, e.g., forabout 10 to 60 minutes, in a bromine-alcohol solution preferablycontaining about 10 to 20 volume percent bromine. While solutionscontaining lower or higher concentrations of bromine can be used, thelower concentrations necessitate longer etching times to achieve thedesired depth of preferential attack whereas higher concentrations canlead to difficulty of controlling the depth of attack. Such a method canbe used, inter alia, where the metallurgical structure of the powder issubstantially completely austenitic. Where spherical powders of thiscomposition include an oxide surface layer, it is preferred that thebromine-alcohol treatment by preceded by the removal of substantiallyall of the oxide layer, e.g., by treating the powder in an acidsolution, such as one containing 5 to 15 parts water, 5 to 15 partsconcentrated (38%) hydrochloric acid and 1 part concentrated (70%)nitric acid. It is generally preferred that such spherical powders be sopre-treated with acid where the powders include more than 500 p.p.m.oxygen, or even 400, 250, or 100 p.p.m. oxygen where the powder meshsize is l00, +200. The water HCI HNO solution preferably is at atemperature of about 40 to 60C., the etching time depending on theamount of oxide present, e.g., about 1 to 60 minutes. After thetreatment to remove the surface oxide, the powder can be rinsed, e.g.,in water or alcohol, and dried, and then trated with the bromine-alcoholsolution, after which the powders can be compacted at, e.g., 20 or 40t.s.i. and then sintered at about 2,000 to 2,100F., for example.

Of course, with a constant oxide thickness, the oxygen level of a powderwill increase with increasing surface area of the powder and, therefore,with decreasing particle diameter. To illustrate, a powder having anaverage particle radius of about 30 microns has a surface area abouttwice that of a comparable volume of powder with an average particleradius of about 55 microns. Among the corrodents that can be used toselectively chemically attack stainless steel powders are the following:1 volume nitric acid in solution in 3 volumes hydrochloric acid; a 10%solution of chromic and hydrochloric acids in water, the amount ofchromic acid being increased for more severe attack; ferric chloride,saturated in hydrochloric acid, including a small percentage of nitricacid; 4 parts by weight cupric sulphate and 20 parts by weighthydrochloric acid in solution in 20 parts by weight of water; and asolution of 50% hydrochloric acid in alcohol. Where it is desired,selective attack can be carried out with acid solutions containing,e.g., ferric chloride or copper chloride, so as to achieve localizedpitting of the powder.

Where it is desired, powder containing ferrite and another phase, e.g.,austenite or martensite, can be selectively attacked with a solution of,by weight, 5 parts cupric chloride, parts hydrochloric acid, 100 partsethyl alcohol, and 100 parts water, such corrodent attacking the ferritemore than austenite but less than martensite.

In general, the initial size of the powder particles that are used inpracticing the invention is determined by the properties that are soughtin the sintered compact. However, a relatively coarse powder, e.g.,about 500 microns or larger is generally undesirable because a very deepattack, e.g., 50 microns, which is difficult to achieve, would berequired to achieve the degree of surface deformation necessary forrapid sintering of the compacted powders. Also, the required very deepattack would result in a powder compact with poor appearance. On theother hand, too fine an initial powder particle will result in thecomplete dissolution of many particles or make it difficult to achieveselective attack on a scale fine enough relative to the particle size,to permit ready compaction of the powders. For these reasons, an averagepowder particle size of about 50 to about microns is preferred.

Generally, a relatively deep selective attack will not provide any largegain in compactibility but will merely be a waste of metal andcorrodent. On the other hand, too shallow a selective attack on theparticles will not provide any significant improvement in compactibilityand will necessitate longer sintering times. In general, the depth ofattack preferably is about 5 to about 15 microns for powder particlesizes of 40 to 400 microns diameter. Also, the attack should be on asufficiently fine scale, that is, the less chemically resistant surfaceportions should be uniformly distributed and relatively close togetherbut separated by the more resistant surface regions, so that the maximumnumber of asperities can be produced, thereby promoting a relativelyhigh degree of interlocking among the treated particles during thecompaction process.

During the step of selectively attacking the particles, the particlesremain substantially unfragmented with only the occasional very smallpowder particles being completely dissolved, the larger particlesremaining substantially whole except for the selectively removed surfaceregions. The selectively attacked powder particles substantially retaintheir spherical configuration but contain deformable microscopicasperities at their surfaces.

In addition to stainless steel powders, the present infor one-half hourat 1,700F. in a hydrogen atmosphere to produce in the various powderparticles a dispersed austenite phase in a matrix of ferrite. Portionsof each annealed sieve fraction were then immersed in a boiling ventionis applicable to the treatment of spherical nick- 5 percent sulfuricacid solution in water, for times el-base super-alloy powderscontaining, by weight, varying from 5 to 60 mmutes, after which thepowders about 10 to about chromium, up to about cowere washed in alcoholand dried by warm arr. The varbalt, up to about 25% molybdenum, up toabout 10% ious acid-treated portions were then pressed in a dietungsten, up to about 6% columbium, up to about 5% having a cavity withcross-sectional dimensions of onealuminum, up to about 5% titanium, upto about 20% to half inch by 1% inches. The pressure that was appliediron, up to about 1% manganese, up to about 1% silit0 the variouspowders was either 20 or tons per con, up to about 0.25% carbon, and thebalance nickel q a Inch Afterthe Powders were pre ey e e and incidentalimpurities. Such nickel-base super-alloy Studled t0 detfitmme l degreeof compactlon, If y, powders include austenitic matrices, which can besethat achieved, this being measured y the green lectively attackedchemically, e.g., by a brominel5' y of the p containing solution, suchas an alcohol-l0 to 20 volume Sinteri'ng was conducted for one-half hourat bromine solut1o n, or by a strong oxidizing acid, such 2,050F. in ahydrogen atmosphere. The results obas concentrated nitric acid. tainedwith the various treated powders are compared Selective chemical attackon such nickel-base superin Table I below with a portion (Powder No. l)of the alloy powders can be enhanced by heat treating the 20 sameatomized powder that was annealed in the same powders at, e.g., aboutl,400 to 2,000 F. to produce way but not subjected to chemical attack.

TABLE I Time in Compacting Particle Average Density Powder BoilingPressure Size of Theoretical) No. H SO (min.) (tsi) Range Green Sinteredl O 40 100,+200 Loose Powder No Compacting 2 5 20 1o0,+200 SlightCompacting 3 5 20 '20o,+325 Slight Compacting 4 1O 20 l00,+200 SlightCompacting 5 1O 20 200,+325 Moderate Compacting 6 30 20 -100,+200 65.566.6 7 30 20 200,+325 Compact Cracked 8 30 40 l00,+200 75.5 76.2 9 3o 40200,+325 Compact Cracked I0 60 20 -100,+200 67.3 67.7 It 60 20 -200,+32565.8 66.5 12 6O 40 200,+325 74.9 75.3

therein a second phase such as gamma prime precipi- From the table, itcan be seen that compaction was t m not achievable with the untreatedpowder even where Where the spherical, nickel-base super'alloy powde theapplied pressure was 40 tsi. The light acid attack on contains asubstantial amount of chromium, e.g., about the P de de gnated asNumbers 2 through 5 pro- 15 to about 25 weight percent, there can beformed at duced y shght to moderate p t n. the ornthe powder surfaces apassive film that can be broken p c Of these p wders cracking onhandling so that smdown locally at the more active areas by using, e.g.,a terlng not camed out f Powder P 1 through solution containing halogenions, such as HCl. The 10- A fairly deep attack achieved y etching thfiT100, calized breakdown of the passive film results in the exmesh Powdercllts 6 and for 30 minutes posure of parts of the underlying metalsurface, which f the compact?" of these P F at 20 40 exposed parts canthen be attacked to the desired depth the Tesultmg colnPacts having ledges by the halogen ions or other suitable col-rodent and sufficientstrength to withstand handling. The 30 A M W. V V minute etch of the200, +325 powder sieve fractions EXAMPLE (numbers 7 and 9) allowed thesepowders to be comt pacted but the compacts cracked during the pressing Astifunless Steel Powder havmg t Composition of, operation so that nosintering was carried out for these y Weight, 01% carbon, 80% nickel,275% chfopowders. A 60 minute etch of both powder sieve fracm m, xyg n,72 nitr gen, and the baltions,l00, +200 mesh and -200, +325 mesh,permitance essentially tron, was produced by atomization of tedcompaction of these powders (Numbers 10, 11, and a corresponding meltcomposition in an argon atmo- 12) at 20 or 40 tsi., the resultingcompacts having sharp sphere, the argon pressure being about 400 p.s.i.The edges and sufficient strength to withstand handling. atomizedpowder, the various particles of which were Those powders, i.e., Numbers6, 8 and 10 through 12,

generally spherical, was screened to provide two powder sieve fractions,namely, l00, +200 mesh and -200, +325 mesh. The sieve fractions wereannealed that were compactible were successfully sintered. Though theaverage densities of the various sintered compacts were rather low,microscopic examination revealed that substantial sintering had occurredat the interior of the sintered bodies, the densities at these interiorsbeing estimated to be in excess of 95% of theoretical.

Because the pre-alloyed spherical powders treated according to theinvention exhibit improved compactibility and because compacts thereofexhibit relatively high green density, a lower sintering temperature canbe employed. Such lower sintering temperature, as well as the reducedaccessiblity of the interior regions of the compacts to oxygen,attributable to the relatively high densities, reduce the amount ofoxidation occurring in the chromium-containing alloys.

EXAMPLE II A stainless steel powder composed of, by weight, 0.008%carbon, 0.49% manganese, 022% silicon, 14.7% nickel, 16.8% chromium,1.5% molybdenum, and the balance essentially iron, was produced by argonatomization at 600 psi argon pressure, of a melt of correspondingcomposition. The atomized powder, which was composed of generallyspherical particles was screened to provide a powder fraction of l00,+200 mesh. This powder was then pickled for about 2 minutes in a 50C.solution of parts water 10 parts concentrated HCl 10 parts concentratedHNO to remove surface oxide. The powder was then rinsed in water andthen in alcohol and dried in air. The powder was then treated with asolution of volume percent bromine-alcohol for 10 minutes. The powderwas again rinsed and dried. The etched powder was then pressed at 40tons per square inch and sintered for 1 hour at 2,050F. in crackedammonia. The resulting composition had only 13 percent porosity.

Irregularly shaped powder particles of similar composition, that wereproduced by water atomization were compacted and sintered under similarconditions. The sintered compacts produced from this powder exhibitedhigher porosity, specifically, about 16.5 percent.

Also, electrochemical attack can be employed instead of chemical attack,to roughen the powder surfaces.

The present invention can be employed to produce various stainless steelpowder metallurgy products, including faucet components, marinehardware, including tie-down lugs and capstan components, winches, nutsand brackets.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, asthose skilled in the art will readilyunderstand. Such modifications and variations are to be consideredwithin the purview and scope of the invention and appended claims.

We claim:

1. A process for improving the compactibility of metal powders ofsubstantially spherical configuration and of relatively poor initialcompactibility, comprismg:

i. subjecting said powders to the action of a corrodent whichselectively chemically attacks the less chemically resistant regions ofthe powder surfaces so as to thereby roughen surfaces on a fine scale;

ii. compacting said roughened particles by subjecting the particles to acompressive force sufficient to cause the particles to form aself-sustaining body; and thereafter iii. sintering said body at atemperature of up to about 2,100F. or higher.

2. The process defined in claim 1 in which the roughened particles arecompressed under a force of about 10 to about tons per square inch toproduce a body having a green density of about 65 to about oftheoretical.

3. The process defined in claim 1 wherein said spherical powderparticles are rendered more susceptible to said selective chemicalattack by a preliminary heating step which comprises heat treeating saidpowders to produce therein at least two metallurgical phases, one ofsaid phases being dispersed in a matrix comprising a second one of saidphases.

4. The process defined in claim 1 wherein the metal powder consists ofstainless steel consisting essentially of about 12 to about 35%chromium, up to about 30% nickel, up to about 3% copper, up to about0.5% carbon, up to about 0.5% oxygen, up to about 0.2% nitrogen, up toabout 0.4% sulfur, up to about 0.4% phosphorus, up to about 2% silicon,up to about 10% manganese, up to about 15% cobalt, up to about 10%molybdenum, up to about 5% tungsten, and the balance iron.

5. The process defined in claim 3 wherein the powder consistsessentially of, by weight, about 18 to about 35% chromium, about 2 toabout 12% nickel, up to about 0.2% carbon, up to about 2% manganese, upto about 2% silicon, up to about 2% cobalt, up to about 3% molybdenum,up to about 5% tungsten, and the balance essentially iron, thepreliminary heat treatment comprises annealing the powders so as toproduce therein a metallurgical structure of austenite dispersed in aferrite matrix.

6. The process defined in claim 3 wherein the powder consistsessentially of, by weight, about 20 to about 35% chromium, about 0 toabout 24% nickel, up to about 0.2% carbon, up to about 2% manganese, upto about 2% silicon, up to about 2% cobalt, up to about 3% molybdenum,up to about 5% tungsten, and the balance essentially iron and thepreliminary heat treatment is carried out to produce in the powder twometallurgical phases respectively comprising sigma phase and one ofaustenite and ferrite, the sigma phase being dispersed in a matrix ofone of said ferrite and austenite phases.

7. The process defined in claim 1 wherein the powder particles have aninitial average diameter of about 40 to 400 microns and the selectivechemical attack is carried out to a depth of about 1 to about 20microns, the compacting being carried out at a pressure of about 10 toabout 70 tons per square inch to produce a body having a green densityof about 65% to about 85% of theoretical.

8. The process defined in claim 1 wherein the chemical attack is carriedout so as to remove the surface regions of said particles to a depthsufficient to weaken the surface portions thereof, thereby allowingready deformation at the surfaces of said particles.

9. The process defined in claim ll wherein the selective chemical attackcomprises treating said spherical particles in a bromine-alcoholsolution.

10. The process defined in claim 9 wherein the spherical powderparticles include oxides at the surfaces thereof and the selectivechemical attack is preceded up to about 10% tungsten, up to about 6%columbium, up to about 5% aluminum, up to about 5% titanium, up

' to about 20% iron, up to about 1% manganese, up to about 1% silicon,up to about 0.25% carbon, and the balance nickel.

1. A PROCESS FOR IMPROVING THE COMPACTIBILITY OF METAL POWDERS OFSUBSTANTIALLY SPHERICAL CONFIGURATION AND OF RELATIVELY POOR INITIALCOMPACTIBILITY, COMPRISING: I. SUBJECTING SAID POWDERS TO THE ACTION OFA CORRODENT WHICH SELECTIVELY CHEMICALLY ATTACKS THE LESS CHEMICALLYRESISTANT REGIONS OF THE POWDER SURFACES SO AS TO THEREBY ROUGHENSURFACES ON A FINE SCALE; II. COMPACTING SAID ROUGHENED PARTICLES BYSUBJECTING THE PARTICLES TO A COMPRESSIVE FORCE SUFFICIENT TO CAUSE THEPARTICLES TO FORM A SELF-SUSTAINING BODY; AND THEREAFTER III. SINTERINGSAID BODY AT A TEMPERATURE OF UP TO ABOUT 2,100*F. OR HIGHER.
 2. Theprocess defined in claim 1 in which the roughened particles arecompressed under a force of about 10 to about 70 tons per square inch toproduce a body having a green density of about 65 to about 85% oftheoretical.
 3. The process defined in claim 1 wherein said sphericalpowder particles are rendered more susceptible to said selectivechemical attack by a preliminary heating step which comprises heattreeating said powders to produce therein at least two metallurgicalphases, one of said phases being dispersed in a matrix comprising asecond one of said phases.
 4. The process defined in claim 1 wherein themetal powder consists of stainless steel consisting essentially of about12 to about 35% chromium, up to about 30% nickel, up to about 3% copper,up to about 0.5% carbon, up to about 0.5% oxygen, up to about 0.2%nitrogen, up to about 0.4% sulfur, up to about 0.4% phosphorus, up toabout 2% silicon, up to about 10% manganese, up to about 15% cobalt, upto about 10% molybdenum, up to about 5% tungsten, and the balance iron.5. The process defined in claim 3 wherein the powder consistsessentially of, by weight, about 18 to about 35% chromium, about 2 toabout 12% nickel, up to about 0.2% carbon, up to about 2% manganese, upto about 2% silicon, up to about 2% cobalt, up to about 3% molybdenum,up to about 5% tungsten, and the balance essentially iron, thepreliminary heat treatment comprises annealing the powders so as toproduce therein a metallurgical structure of austenite dispersed in aferrite matrix.
 6. The process defined in claim 3 wherein the powderconsists essentially of, by weight, about 20 to about 35% chromium,about 0 to about 24% nickel, up to about 0.2% carbon, up to about 2%manganese, up to about 2% silicon, up to about 2% cobalt, up to about 3%molybdenum, up to about 5% tungsten, and the balance essentially ironand the preliminary heat treatment is carried out to produce in thepowder two metallurgical phases respectively comprising sigma phase andone of austenite and ferrite, the sigma phase being dispersed in amatrix of one of said ferrite and austenite phases.
 7. The processdefined in claim 1 wherein the powder particles have an initial averagediameter of about 40 to 400 microns and the selective chemical attack iscarried out to a depth of about 1 to about 20 microns, the compactingbeing carried out at a pressure of about 10 to about 70 tons per squareinch to produce a body having a green density of about 65% to about 85%of theoretical.
 8. The process defined in claim 1 wherein the Chemicalattack is carried out so as to remove the surface regions of saidparticles to a depth sufficient to weaken the surface portions thereof,thereby allowing ready deformation at the surfaces of said particles. 9.The process defined in claim 1 wherein the selective chemical attackcomprises treating said spherical particles in a bromine-alcoholsolution.
 10. The process defined in claim 9 wherein the sphericalpowder particles include oxides at the surfaces thereof and theselective chemical attack is preceded by a step comprising the treatmentof the spherical powder with an acid solution so as to remove the oxidessubstantially completely.
 11. The process defined in claim 1 wherein themetal powder is of a nickel-base superalloy consisting essentially of,by weight, about 10 to about 25% chromium, up to about 30% cobalt, up toabout 25% molybdenum, up to about 10% tungsten, up to about 6%columbium, up to about 5% aluminum, up to about 5% titanium, up to about20% iron, up to about 1% manganese, up to about 1% silicon, up to about0.25% carbon, and the balance nickel.